1. Field of the Invention
This invention relates to vacuum or gas-filled valve devices in which electrons are emitted from a cathode by virtue of a field emission process.
2. Description of Related Art
Field emitter electron sources produced by micro-fabrication techniques have a number of potential advantages over thermionic cathodes. Firstly, thermionic cathodes require a substantial amount of cathode heating power, which is not required by field emission sources. More especially, field emitters are capable of providing electron beams which exhibit a lower energy spread, greater uniformity and greater current density, all of which can be obtained at low voltage.
In order to achieve these capabilities, however, it is necessary to fabricate many emitters of nanometer scale uniformly over macroscopic areas.
A basic structure of a known field emitter electron source comprises, an electrically-conductive pyramid or conical shape or "tip", projecting from a substrate. There may be many such tips, for example 10.sup.6 or 10.sup.8, on a single 10 cm diameter silicon substrate.
There are various known microfabrication methods for producing such tips. For example, British Patent Publication No. 2,209,432 discloses the production of a tip (which may be one of many tips formed in a single process), depositing an insulating spacer layer and a grid layer over the tip or tips and then defining and producing a grid aperture over the or each tip by a lithographic process. Such process requires arcuate alignment of each grid aperture relative to the tip. The requirement to achieve such accuracy tends to reduce the yield of the process. U.S. Pat. No. 3,755,704 and European Patent No. 0345148 disclose the provision of a lithographically-defined grid structure through which the tips are deposited by evaporation. British Patent No. 1,583,030 discloses the formation of a grid on an array of tips formed in a unidirectional solidified eutectic. Neither of these methods requires any specially accurate alignment of separate lithographic process steps. The first method involves only one essential lithographic process, but the tips must be formed by an evaporation process. The second method requires no lithographic processes, but requires a specific, namely eutectic, form of tip material.